This invention relates to a mechanical dock leveler and in particular to an improved actuation and latch mechanism.
Dock levelers or dockboards are adapted to span a gap between a loading dock and the bed of a carrier. These devices include a frame or supporting structure that is mounted in a pit of the dock or on its own frame. The leveler is adapted to be moved from a generally horizontal position, the cross-traffic position, where it lies flush with the upper surface of the dock to a second generally inclined position in which, together with an outwardly extending lip, provides a ramp which bridges the distance between the bed of the carrier and the dock surface.
The art is replete with the number of such structures. They are typically actuated by springs, hydraulics or the like. Typical is a spring loaded dock board illustrated in U.S. Pat. No. 3.137.017. Hydraulically actuated system as illustrated in U.S. Pat. Nos. 4.619.008 and 4,955,923. Representative of other art in the technology relating to techniques of actuation of the dock board and/or the lip are found in U.S. Pat. Nos. 3,299,456; 3,368,229; 3,530,488; 3,835,497; 3,858,264; 3,877,102; 3,995,342; 4,126,909; 4,279,050; 4,328,602; 4,455,703 and 4,922,568. Those patents all illustrate a myriad of techniques for raising and lowering dock boards including the use of safety legs, stops and the like.
A problem on mechanically actuated dock levelers is that the deck weight is supported by springs which propel the deck upward when the hold down device is released. The lip is extended by a linkage, typically a chain attached to the lower frame. The chain tightens as the deck reaches the top of its travel. U.S. Pat. No. 3,137,017 illustrates such a mechanism utilizing a cable coupled with a lever mechanism to raise the lip from its pendant position to an outwardly extending position as the dock board is raised. The lip is usually counter balanced so that it can be easily extended but is held in the extended position by a latch or other mechanism until the leveler is "walked down" to a truck bed. That is, once the deck has been raised to its maximum position. the operator will walk on the leveler to lower it to a proper position where the lip makes contact with the bed of the carrier. The lip is then supported by the truck and the latch falls away.
Due to wear and other environmental considerations when taken with a lack of lubrication or spring fatigue, mechanical dock levelers have a tendency to lose the ability to fully extend the lip. When such occurs, the lip will not latch, that is, be held in an outward position but tends to fall back to the pendent position. This function can be restored by a periodic preventive maintenance and the like and/or adjustment of the springs. But until such occurs the leveler is inoperative. As can be appreciated, these devices operate in a heavy industrial context such as shipping docks or the like.
To overcome this problem some levelers have the mechanical latch replaced by a viscous damper such as an hydraulic shock absorber. This allows free extension of the lip and restricted retraction. If the lip does not fully extend then the shock absorber will still retard the lip long enough for the operator to walk the leveler down to the truck bed.
However, there are two significant draw backs with the use of the shock absorber. First, a mechanical latch will keep the lip extended as long as desired but the shock absorber will begin to retract as soon as the load is applied to it thus forcing the operator to walk the leveler immediately down. Secondly. the viscosity of the hydraulic fluid in the shock absorber is sensitive to changes in temperature. That is. in warm temperatures the lip will fall quickly forcing the operator to walk the leveler down quickly before the lip falls too far to have necessary span for it to rest on the truck bed. Conversely. in cold temperatures the lip will fall very slowly thus forcing the operator to wait unnecessarily a long time for the lip to fall when attempting to remove the leveler from the truck.
The hydraulic shock absorber does however have one advantage which is difficult to achieve in systems that heretofore employed mechanical latches. That is. the hydraulic shock absorber has the ability to retract quickly under high load which may protect the lip mechanism from damage when it is accidently struck by a truck which backs into the lip while still extended.